1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens, and more particularly, though not exclusively, a zoom lens that can be used, for example, in an image pickup apparatus.
2. Description of the Related Art
Recently, the market has desired a highly functional and small-sized image pickup apparatus (camera), such as a video camera or a digital still camera. Furthermore, the market has also desired a small optical system (imaging optical system), which is used in such a camera, having a wide angle of view and a large aperture, with a small number of lenses, and having a high optical performance.
A camera using an image sensor includes a low-pass filter and a color collection filter disposed between a rearmost lens portion and the image sensor. Accordingly, it is required that a zoom lens for use with such a camera has a relatively long back focal length.
In addition, in a camera using an image sensor for a color image, in order to prevent shading in luminance and color, it is desired that a zoom lens for use with such a camera is telecentric on the image side.
A so-called retro focus type zoom lens having a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power in order from the object side to the image side is known as a zoom lens that is telecentric on the image side.
As one of such retro focus type zoom lenses, U.S. Pat. Nos. 6,545,819 and 6,498,687 each discuss a three-unit zoom lens including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power in order from the object side to the image side.
In addition, U.S. Pat. No. 5,009,491 and U.S. Patent Application Publication No. US 2004/0150890 A1 each discuss a three-unit zoom lens capable of correcting various aberrations occurring due to a high zoom ratio with a third lens unit.
Moreover, Japanese Patent Application Laid-Open No. 2004-318104 and U.S. Pat. No. 7,042,651 each discuss a small-size three-unit zoom lens having a first lens unit including two lens elements.
In recent years, attempts have been made to both implement the downsizing of a camera and increase the zoom ratio of a zoom lens that is used for the camera. The method for both implementing the downsizing of a camera and increasing the zoom ratio includes a so-called lens retraction method. In the lens retraction method, the interval between lens units in a non-photographing state is reduced to an interval that is different from the interval in a photographing state so as to reduce the amount of protrusion of the lens from the camera body.
As the number of lens elements of each lens unit that constitutes a zoom lens is large, the length of each lens unit along an optical axis becomes large and, thus, the entire length of the zoom lens becomes large. In addition, when the amount of movement of each lens unit during zooming or focusing is large, the entire length of the zoom lens becomes large. As a result, a desired length of the zoom lens with the lens units retracted cannot be obtained. Accordingly, it becomes difficult to utilize the lens retraction method. That is, as the zoom ratio of a zoom lens becomes higher, the entire length of the zoom lens becomes larger, and accordingly, it becomes difficult to apply the lens retraction method.
If the number of lenses in each lens unit or the total number of lenses in the entire zoom lens is merely reduced to reduce the total length of the lenses at the time of retraction, it becomes difficult to correct aberrations, such as spherical aberration and coma, which concern a monochromatic image forming performance. Furthermore, since only a limited range of glass material can be used, it becomes highly difficult to correct or reduce chromatic aberration.
In the optical design field, the number of lenses in the entire optical system can be reduced often by using a lens having an aspheric surface. However, in the case of using an aspheric lens, aberrations related to the monochromatic image forming performance can be corrected, but it is difficult to correct or reduce chromatic aberration occurring mainly due to selection of a glass material.
In addition, as one of methods for processing a lens surface to produce an aspheric surface, there is known a method of laying a resin on a lens surface of a base lens and pressing an aspheric-shaped mold against the resin layer to form an aspheric lens. With this method, the entire length of the lens can be shortened while correcting or reducing aberrations with an aspheric lens. However, it is difficult to correct or reduce chromatic aberration unless a setting based on the difference between a material of the lens and a material of the resin is appropriately defined.
In particular, if the lens configuration of a first lens unit, whose effective diameter is generally large, is inappropriately set, it is difficult to obtain a high optical performance while downsizing the entire optical system.